Magnitude programming for implantable electrical stimulator

ABSTRACT

A neural stimulation system allows the magnitude of electrical stimuli generated by the system to be programmed to a desired level greater than or equal to a minimum perceived threshold and less than or equal to a maximum tolerable perceived threshold. The electrical stimuli are applied through selected groupings of individual electrode contacts of a multi-electrode-contact electrode array attached to pulse generation circuitry as either cathodes or anodes. The electrode array is implanted so that the individual electrode contacts are in contact with the body tissue to be stimulated. Stimulating electrical current pulses, defined by a prescribed set of stimulus parameters are generated and applied to the selected electrode contacts so as to flow from the anode electrodes to the cathode electrodes. The perceived magnitude of the applied stimuli is equalized in order to enable quick, automated, and/or interactive programming of the values of the stimulation parameters.

This application claims the benefit of U.S. Provisional Application Ser.No. 60/172,167, filed Dec. 17, 1999, which application is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to implantable stimulators, e.g., animplantable neural stimulator, and more particularly to a system andmethod for programming the magnitude (e.g., amplitude) of thestimulation pulses that are generated by such neural stimulator. Theinvention may be used with a wide variety of neural stimulators, e.g.,spinal cord stimulators, brain stimulators, urinary incontinencestimulators, cochlear stimulators, and the like.

Neural stimulation systems of the type with which the present inventionmay be used are described in the art, e.g., in U.S. Pat. No. 5,603,726.Such stimulator systems typically include: (1) an implantable pulsegenerator; (2) an electrode array connected to the implantable pulsegenerator; and (3) some means, e.g., an external programmer, forcontrolling or programming the implantable pulse generator. Inoperation, the implantable pulse generator generates an electricalstimulation pulse, or pulse sequence, in accordance with a prescribedpattern or stimulation strategy. Each pulse may be programmed or set toa desired magnitude (amplitude and/or pulse width), and applied at a setor programmed rate (or frequency) to surrounding body tissue through aselected pair or grouping of electrode contacts of a multiple electrodearray.

U.S. Pat. No. 5,895,416 teaches one type of method and apparatus forcontrolling and steering an electric field. The '416 patent steers thesize and location of the electric field in order to recruit only targetnerve tissue and exclude unwanted nerve tissue. Such steering is done bychanging the voltage amplitude at each anode in response to changes inelectrode impedance of an electrode array in order to maintain aconstant anodic current.

Unfortunately, programming implantable stimulators that havemulti-electrode contacts can be a very time consuming task. This isparticularly true for a spinal cord stimulation system, or similarsystem, where there are typically 8 to 16 electrode contacts on theelectrode array through which the stimulation pulses are applied to thespinal nerves. With 8 to 16 electrode contacts there are thousands andthousands of possible electrode combinations. The goal of programming aspinal cord stimulator, or similar neural stimulator, is to figure outwhich electrode combinations of the thousands that are possible shouldbe used to apply electrical stimulus pulses (each of which cantheoretically be programmed to have a wide range of amplitudes, pulsewidths, and repetition rates) so as to best serve the patient's needs.More particularly, the goal of programming a spinal cord stimulator, orother neural stimulator, is to optimize the electrode combination ofanodes and cathodes, as well as the amplitude, pulse width and rate ofthe applied stimulation pulses, so as to allow the stimulator to bestperform its intended function, e.g., in the case of a spinal cordstimulator, to relieve pain felt by the patient. The manual selection ofeach electrode combination and the stimulus parameters that are usedwith such electrode combination (where the “stimulus parameters” includeamplitude, pulse width, and repetition rate or frequency) is anunmanageable task. What is needed is a system and method for programminga neural stimulator, such as a spinal cord stimulator, that automatesmuch of the programming process using interactive programs, and inparticular wherein the amplitude of the applied stimulus may beprogrammed in a way that facilitates the use of automated andinteractive programs safely and effectively.

A disadvantage associated with many existing neural stimulators is thatsuch systems cannot independently control the amplitude for everyelectrode in a stimulating group of electrodes, or “channel”. Thus,stimulation fields cannot be highly controlled with multiple stimulatingelectrodes. Instead, a constant voltage is applied to all electrodesassigned to stimulate a target site at any given time (wherein theelectrodes thus assigned are referred to as a “channel”). While suchapproach may make programming such stimulator a much more manageabletask (because the number of possible electrodes and parameter choicesare severely limited), such limitations may prevent the stimulator fromproviding the patient with an optimal stimulation regimen. What isneeded, therefore, is a neural stimulator wherein all of the possibleelectrode combinations and parameter settings can be used, and wherein aprogramming technique exists for use with such neural stimulator wherebyan optimum selection of electrode combinations and parameters settingsmay be quickly and safely identified and used.

To illustrate the problem that a clinician or other medical personnelfaces when programming a typical neural stimulator wherein eachelectrode on an electrode array may be tested, consider the followingexample. The clinician typically performs an electrode test in terms ofthe patient's response, e.g., by selecting a set of electrodes(assigning anodes and cathodes), setting a pulse width and rate, and bythen increasing the amplitude until the patient begins to feelstimulation. The clinician continues to increase the stimulationamplitude until the stimulation is strongly felt, and then the patientis asked the location where the stimulation is felt. The stimulation isthen turned off and the steps are repeated for the next electrode set,until the clinician has a good map of the stimulation coverage by anelectrode array. Disadvantageously, however, the clinician cannot simplyjump from one set of electrodes within the array to a next set ofelectrodes within the array and back again while the stimulation isturned on because the perception thresholds vary. What is needed is asystem or apparatus that does not require that the clinician turn offthe stimulation and start over, increasing the amplitude for eachelectrode set, but rather allows the clinician to compare back and forthbetween electrode sets while continuously obtaining patient feedback.What is further needed is a system or apparatus wherein the cliniciandoes not have to start over every time he or she repeats aformerly-tested electrode set. Unfortunately, with current programmingapproaches, because there is no means to automatically adjust themagnitude of stimulation, the clinician must start over every timeanother electrode set is selected.

The threshold ranges derived from such clinician tests usually vary fromone electrode to the next. That is, at a given pulse width theperception threshold at a first electrode may be 2 milliamps (mA), (or 2volts (V) at 1000 ohms), with a maximum tolerable threshold of 7 mA (or7 V at 1000 ohms). At that same pulse width, the perception threshold ata second electrode may be 3 mA (or 3 V at 1000 ohms), with a maximumtolerable threshold of 6 mA (or 6 V at 1000 ohms). Thus, switching fromone electrode to the next at a constant current or voltage level may notbe done without the patient feeling perceptual intensity differences.Moreover, automatically switching between electrodes at constantparameter outputs could result in unintentionally exceeding the maximumtolerable threshold associated with a particular electrode where themaximum tolerable threshold of the electrode is less than the maximumtolerable threshold of another electrode. As a result, the typicalmethod for testing electrode combinations, as described above, is tostart with the output at zero for every combination of electrodes(including combinations which result in too many combinations to test),and gradually increase the amplitude to a comfortable level, and thenhave the patient respond to paresthesia or other coverage. No quick orrapid electrode switching can be done. It is thus evident that what isneeded is a system and method of equalizing the perceived amplitude, andto thereby enable quick, automated, and/or interactive (i.e.,directional programming) methods.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing asystem and method for programming the magnitude of electrical stimuligenerated and applied by a neural stimulator. e.g., a spinal cordstimulator. The electrical stimuli are applied through selectedgroupings of individual electrode contacts of a multi-electrode-contactelectrode array attached to pulse generation circuitry as eithercathodes or anodes. The electrode array is implanted so that theindividual electrode contacts are in contact with the body tissue to bestimulated. Stimulating electrical current pulses, defined by aprescribed set of stimulus parameters, e.g., pulse amplitude, pulsewidth and pulse repetition rate, are generated and applied to theselected electrode contacts so as to flow from the anode electrodes tothe cathode electrodes. As the current pulses flow through the bodytissue, the electrical current causes the neural stimulator to carry outits intended function, e.g, triggering a desired neural response orblocking an undesired neural activity. The present inventionadvantageously provides a programming system or method whereby theperceived magnitude of the applied stimuli is equalized in order toenable quick, automated, and/or interactive selection of the stimulationparameter values that are used by the stimulator.

In accordance with one aspect of the invention, the invention may thusbe characterized as a method of operating an implantable neuralstimulator. The implantable neural stimulator with which such method isused typically includes a programmable pulse generator circuit that ishoused within a sealed case. An electrode array is connected to theneural stimulator. The sealed case may function as a reference electrodefor some electrode configurations. The electrode array has a pluralityof electrode contacts, identified as electrodes 0, 1, . . . n, where nis an integer of at least one (so that electrodes 0 and 1 comprise theat least two electrodes). In one embodiment, for example, n may be aninteger of, e.g., three (so that there are four electrodes, electrodes0, 1, 2 and 3). In another embodiment, n may be an integer of, e.g., 15(so that there are sixteen electrodes, electrodes 0, 1, 2, . . . 15).The invention may be used with any number n of electrodes.

The method of operating the neural stimulator, broadly stated,comprises: (a) measuring and recording at least one perception point,e.g., a minimum perception threshold level or a comfortable thresholdlevel, for all of the electrode contacts and combinations thereof, or(where the number of electrode contacts and combinations thereof is toolarge to measure) measuring at least one perception point for a selectedsubset of the electrode contacts and combinations; (b) estimating andrecording perception points for unmeasured electrode contacts or sets ofelectrode contacts; and (c) mapping the recorded measured and/orestimated values to magnitude levels for use with other designatedstimulation parameters.

In accordance with another aspect of the invention, the invention may becharacterized as a neural stimulation system wherein an optimal set ofstimulus parameters may be determined and programmed into the system.Such system includes: (a) an implantable neural stimulator comprising asealed case, and having pulse generation circuitry contained inside ofthe case that is adapted to generate a stimulus pulse in accordance withprogrammed stimulus parameters; (b) an electrode array having amultiplicity of electrode contacts, wherein each of the multiplicity ofelectrode contacts may be selectively connected to the pulse generationcircuitry within the implantable neural stimulator; (c) a knownthreshold level, e.g., a known perception threshold level, a knowncomfortable threshold level, and/or a known maximum tolerable thresholdlevel, for at least a plurality of the multiplicity of electrodecontacts, where such known threshold levels are typically expressed inunits of current or voltage; and (d) an equalizer circuit or system ortechnique (hereafter referred to simply as an “equalizer”) thatequalizes the threshold level(s) to unit-less magnitude levels. In apreferred embodiment, the equalizer further estimates correspondingunit-less magnitude levels for any electrode contacts, or combination ofelectrode contacts, for threshold levels not initially known ormeasured. Once the unit-less magnitude levels are known or estimated foreach electrode within a given electrode set, the neural stimulationsystem may thereafter automatically set or adjust the magnitude of thestimulus applied through the given electrode set when such givenelectrode set is selected as the electrode set through which stimulationis to be applied. Advantageously, such neural stimulation system allowsstimulation to be applied through different electrode sets withouthaving to ramp the amplitude up from a zero value for each electrode setselected. Hence, a clinician using such neural stimulation system mayimmediately jump between two or more electrode sets, each of which hasthe stimulation magnitude levels automatically adjusted to, e.g., aminimum perception threshold level, a maximum tolerable threshold level,or a selected value between the minimum perception and maximum tolerablethreshold levels, as different stimulus parameters are tested.

Any of numerous techniques and approaches known in the art may be usedto measure perception threshold and maximum tolerable thresholds.Similarly, any one of numerous stimuli-application circuits and testingtechniques known and practiced in the art may be used for applying astimulus of a prescribed magnitude to one or more electrodes and testingwhether such application produces a desired result. For an SCS system,the desired result will typically involve determining, e.g., whether theresulting paresthesia sensed by the patient sufficiently blocks painfelt by the patient.

The “equalizer” referenced in the system described above is at the heartof the present invention. Such equalizer advantageously equalizes theperception threshold measurement and the maximum tolerable thresholdmeasurement to unit-less magnitude levels for all of the electrodecontacts such that level x of one electrode configuration has an equalintensity perception as another electrode configuration set at level x.Once such equalization has been performed, the magnitude levels maythereafter be used to set the magnitude of the stimulation levelswithout the need to manually ramp up or ramp down the stimulus magnitudeat each electrode contact, as is required in the prior art.

The equalizer may be realized in software, firmware, or hardware.Typically, equalization is performed by (i) storing the perceptionthreshold measurement and the maximum tolerable threshold measurementfor each electrode contact; (ii) mapping the perception thresholdmeasurement of each electrode contact to a first number magnitude level;and (iii) mapping the maximum tolerable threshold measurement to asecond number magnitude level. A stimulus having the first numbermagnitude level causes an electrode contact receiving that stimulus toreceive a stimulus having a magnitude equal to the perception thresholdmeasurement for that electrode contact. Similarly, a stimulus having thesecond number magnitude level causes an electrode contact receiving thatstimulus to receive a stimulus having a magnitude equal to the maximumtolerable threshold measurement for that electrode contact. In apreferred implementation, the first number magnitude level is assignedto be a low value, e.g., “1”, and the second number magnitude level isassigned to be a higher value, e.g., “10”. A stimulus having a magnitudelevel greater than “1” and less than “10” causes a stimulus to beapplied that is somewhere between the perception threshold and maximumtolerable threshold. Typically, the stimulus magnitude increases as themagnitude level number (which is a unit-less number) increases. That is,a magnitude level “m” causes a stimulus to be generated that has amagnitude greater than a stimulus having a magnitude level “m−1”, wherem is an integer of from 1 to 10, and where a magnitude level “0”stimulus comprises a stimulus having zero magnitude.

Where there are a large number of electrode contacts, making itimpractical to measure threshold levels for all possible combinations ofthe electrode contacts, only a subset of the possible electrode contactsneed to be measured, and thereafter the unit-less magnitude levels forthe non-measured electrode contacts may be estimated. For example, itthere are sixteen electrode contacts arranged in line within a lineararray, it would typically only be necessary to measure threshold levels,and assign corresponding unit-less magnitude levels, for the twoelectrode contacts at the edges of the array. Unit-less magnitude levelsmay then be estimated for the electrode contacts that reside between theouter two electrode contacts using an appropriate estimation algorithm,such as a linear extrapolation.

Further, it is not always necessary to measure, for a given electrodeconfiguration for which a measurement is being made, both the minimumperception threshold level and the maximum tolerable threshold level.Rather, in many instances, all that is required is to measure a singlethreshold level, e.g., a comfortable threshold level, from which singlethreshold measurement a corresponding unit-less magnitude thresholdlevel may be assigned, and from which other unit-less magnitude levelsmay be estimated for electrode contacts similarly configured. Forexample, where the unit-less magnitude levels range from “1”(corresponding to the minimum perception threshold level) to “10”(corresponding to a unit-less magnitude level “10”), a comfortableunit-less magnitude level “5” could be equalized with a proportionaterange approximated around the level “5” to give a reasonable operatingrange. This advantageously reduces the number of measured pointsrequired, but disadvantageously is less accurate.

A principle advantage offered by the invention is the ability todynamically switch between electrode sets while the stimulation iscontinuously applied. Such advantage allows immediate comparisons ofstimulation to be made without having to reset the stimulationmagnitude, and while maintaining a relatively constant perception ofintensity or magnitude of stimulation, thereby avoiding over or understimulation.

Further, the invention does not require that threshold measurements betaken for all possible electrode configurations. Rather, a subset of thepossible electrode configurations may be measured, and from suchmeasurements estimates may be made of the unmeasured thresholds withrelative accuracy. For example, unit-less threshold levels for bipolar,monopolar, tripolar, and multipolar combinations may be readilyestimated from actual threshold measurements taken from a subset oftested electrodes.

A feature of the invention is the ability to store adjustments made tostimulation levels for estimated electrode thresholds so that the systemlearns corrections to the estimated equalized levels.

Another feature of the invention is the ability of the system to adjustand compensate the perception stimulus when the pulse width is changed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 shows a simplified block diagram of a neural stimulation system;

FIG. 2 illustrates a typical current stimulation waveform, and definesstimulus parameter values that are typically used to characterize anelectrical stimulus;

FIG. 3A illustrates perception threshold data and maximum tolerablethreshold data obtained from four different electrodes;

FIG. 3B conceptually depicts the manner in which the perception andmaximum tolerable threshold data gathered in FIG. 3A may be mapped intounit-less magnitude “level” settings as part of a magnitude equalizingfunction performed in accordance with the invention;

FIG. 4 is a high level flow chart that depicts a method of programming aneural stimulator in accordance with the invention in order to ascertainoptimum stimulus parameter values that may be programmed into the neuralstimulator;

FIG. 5 is a graph illustrating a representative stimulating currentdistribution for first and second electrodes as a function of amplitude(magnitude) level;

FIG. 6 is a table the quantitatively depicts the data represented in theFIG. 5 in tabular form;

FIG. 7 illustrates patient threshold data in tabular and graphical formfor various electrodes associated with a neural stimulator, and furtherillustrates the concept of current summing when more than two electrodesare included within the stimulation group;

FIG. 8 shows the current and voltage potential distributions obtained atvarious points along an in-line electrode associated with a stimuluspulse that is applied between electrode two (cathode) and electrodeseven (anode) of an in-line electrode having eight electrodes numbered 0through 7; and

FIG. 9 similarly shows the current and voltage potential distributionsobtained at various points along an in-line electrode associated with astimulus pulse that is applied between electrodes one, two and three(cathodes) and electrode seven (anode) of an in-line electrode havingeight electrodes numbered 0 through 7.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

The present invention may be used with many different types of neuralstimulators. By way of illustration, the invention will be described interms of a spinal cord stimulator system (SCS). However, it is to beunderstood that the invention is not limited to use only with a SCS, butmay also be used with other types of multi-electrode neural stimulators,e.g., a deep brain stimulator, a functional electrical stimulator (FES),a cochlear stimulator, or the like.

It is also to be noted that the preferred manner of adjusting themagnitude of the stimulus applied by the neural stimulator is throughadjustment of the stimulus amplitude. However, as is known to those ofskill in the art, amplitude is only one measure of magnitude. Themagnitude of a stimulus may also be adjusted, e.g., by adjusting thewidth or duration of the stimulus pulse, by adjusting the duty cycleassociated with a burst of pulses, by adjusting the repetition rate ofthe applied stimulus (if a single stimulus is applied) or the repetitionrate of the applied burst (if a group of stimulus pulses is applied),and/or by adjusting the width, duty cycle or rate in combination withadjustments of the amplitude in various combinations. Thus, although thepreferred implementation described herein for adjusting the magnitude ofthe applied stimulus is through adjustment of the amplitude and pulsewidth of the stimulus current, such is only exemplary. Any type ofadjustment of the applied stimulus parameters that alters the amount ofenergy delivered to the tissue to be stimulated may be used to practicethe invention, including, e.g., current amplitude adjustment, pulsewidth adjustment, duty cycle adjustment, stimulus frequency adjustment,or combinations thereof.

Turning first to FIG. 1, there is shown a simplified block diagram of aneural stimulation system 10. The stimulation system 10 includes animplantable pulse generator (IPG) 30 connected to an electrode array 40.The electrode array 40 comprises a flexible carrier lead body 41,typically made from silicone rubber, or some other form of Silastic®material, on which individual spaced-apart electrode contacts 42 a, 42b, 42 c, . . . are located near a distal end. There are eight electrodecontacts illustrated in FIG. 1, contacts 42 a, 42 b, 42 c, . . . 42 h.However, this is only exemplary. Any number of electrode contacts may beused. Typically, for a spinal cord stimulator, from four to sixteenelectrode contacts are used. Any number of electrode contacts 42 may beused depending upon the application. At least two electrode contacts,one of which may be included on the case of the IPG 30, must be used toproduce a stimulation.

The array 40 which is shown in FIG. 1 is referred to as an “in-line”electrode array because all of the electrode contacts 42 arespaced-apart and are in line with each other along the body of the leadcarrier body 41. Different types of electrode arrays may be used, e.g.,various forms of paddle arrays, or combinations of in-line arrays.

Each electrode contact 42 a, 42 b, 42 c, . . . 42 h of the electrodearray 40 is connected by way of a wire (not shown in FIG. 1), or otherelectrical conductor, embedded within the lead body to electronic pulsegeneration circuitry housed within the IPG 30. It is through these wiresor conductors that an electrical stimulation pulse is delivered to theelectrode contacts 42.

It is the purpose of the electrode array 40 to place the multiplicity ofelectrode contacts 42 near the body tissue that is to be stimulated. Forexample, in FIG. 1, a nerve fiber 44, or other similar tissue (e.g.,muscle tissue), is shown as being positioned alongside the electrodearray contacts 42. When one of the electrode contacts, e.g., contact 42c, is selected as an anode, and another of the electrode contacts, e.g.,contact 42 h, is selected as a cathode, then a stimulation currentpulse, generated by the IPG 30, may flow along a current path,represented by the dotted line 46, that flows through the body tissueand the nerve fiber 44, thereby stimulating the nerves or other bodytissue that come in contact with the current path. More than oneelectrode contact may be selected as the anode, and/or as the cathode,in such stimulation. Additionally, the IPG 30 may have a referenceelectrode 32 located on its case so that a stimulation current path maybe set up between one or more of the electrodes 42 included within theelectrode array 40 and the reference electrode 32. Such electrodeconfiguration, wherein one of the electrode contacts is the referenceelectrode 32, and one of the electrodes 42 of the electrode array 40 isused as the other electrode, is referred to as monopolar stimulation. Anelectrode configuration where two of the electrodes 42 are used as theelectrodes, one as the anode and one as the cathode, is referred to asbipolar stimulation. An electrode configuration where three of theelectrodes 42 are used as the electrodes, one or two as the anode, andone or two as the cathode, is referred to as tripolar stimulation.Similarly, an electrode configuration where four or more of theelectrodes 42 are used as the electrodes, at least one as the anode andat least one as the cathode, is referred to as multipolar stimulation.

Thus, it is seen that the multiplicity of electrode contacts 42 locatedon the electrode array 40, with or without the reference electrode 32located on the case of the IPG 30, provide a large number of possibleelectrode combinations that could be used when applying stimulationpulses to the body tissue. When all of these possible electrodecombinations are further combined with all of the possible parameterchanges that could be made to the applied stimulus pulse(s), it isevident that the number of combinations that need to be tested in orderto determine an optimum safe and efficacious combination for use by thepatient is almost unmanageable. One purpose of the present invention toprovide a way to quickly and safely identify which electrodecombination, and which stimulus parameters, are most efficacious for agiven patient.

The IPG 30 is controlled by, e.g., programmed by, an external programmer20. The programmer 20, or a component of the programmer 20 (e.g., aprogramming head) is typically placed near the skin 22 of the patient inan area close to the location where the IPG 30 is implanted. Then, asuitable transcutaneous communications link, represented in FIG. 1 bythe wavy arrow 24, is established between the programmer 20 and the IPG30. Such link 24 may take many forms, any of which is suitable forpurposes of the present invention. Exemplary transcutaneous links 24 maybe realized, e.g., through inductive coupling, RF transmission, magneticcoupling, optical coupling, and the like.

Once the transcutaneous link 24 has been established, the programmer 20downloads whatever programming data the IPG 30 needs in order to performits function of generating stimulation pulses. Such data typicallyincludes the stimulus parameters, e.g., pulse width, pulse amplitude,and pulse rate (defined below in connection with FIG. 2), and theindividual electrode contacts 42 through which the electrical stimulusis to be applied to the body tissue. Such data may also include new orupdated operating programs to be used by the IPG as it carries out itsfunction of generating stimulus pulses. In many embodiments of theinvention, the IPG 30 may also send status and/or other data back to theprogrammer 20 through a back telemetry link. When used, the backtelemetry link may be the same as, or different from, the forwardtranscutaneous link 24.

FIG. 2 illustrates a typical stimulation pulse waveform that might beused with the IPG 30. The stimulation pulses shown in the waveform ofFIG. 2 are biphasic pulses, having a first pulse of one polarityfollowed immediately by a pulse of the same magnitude but oppositepolarity. A biphasic pulse offers the advantage of balancing theelectrical charge that flows through the body tissue, which is generallyconsidered to be an important consideration. Other pulse waveforms mayalso be used to provide a charge balanced condition, as is known in theart.

The waveform shown in FIG. 2 is useful, for purposes of the presentinvention, because it defines the common stimulus parameters that areused to define the electrical stimuli generated by the IPG 30. Thesestimulus parameters include, e.g., pulse width (PW), pulse amplitude,and pulse rate. As seen in FIG. 2, the pulse width is the width,measured in units of time (such as microseconds or milliseconds), of theindividual pulses that make up the biphasic pulses. The pulse amplitudeis the magnitude of the peak current in the pulse, measured in units ofelectrical current, e.g., milliamperes (mA). In FIG. 2, the amplitude isshown as “I”. One pulse of the biphasic pulse has an amplitude of +I,and the other pulse has a magnitude of −I. The stimulation rate isdetermined by the inverse of the period T between recurring pulses. Thatis, the rate, expressed in units of pulses per second, or Hz, is equalto 1/T, where T is the time increment between the biphasic (or other)pulses. The pulse rate, depending upon the application, may also beexpressed in units of pulses per minute (PPM). The ratio of the pulsewidth (PW) to the time T (the increment between the pulses) defines theduty cycle.

In accordance with the present invention, once the IPG 30 and theelectrode array 40 have been implanted within the patient, theprogrammer 20, or equivalent device, is used to determine appropriatethreshold levels, e.g., the perception threshold levels and the maximumtolerable threshold levels, associated with an electrical stimuluscurrent applied through a selected set or group of electrode contacts42. Such threshold data typically requires obtaining subjective feedbackform the patient as stimuli of various magnitudes are applied throughvarious electrode combinations.

The threshold data, e.g., perception and maximum tolerable thresholddata, is stored within the programmer 20, or other device having amemory. FIG. 3A illustrates, by way of example, perception thresholddata and maximum tolerable threshold data obtained from four differentelectrodes sets, identified as electrodes sets “a”, “b”, “c” and “d”.Electrode set “a” might comprise, e.g., electrode contact 42 a as acathode combined with reference electrode 32 as an anode (see FIG. 1).Similarly, electrode set “b” might comprise, e.g., electrode contact 42c as an anode combined with electrode contact 42 h as a cathode.Electrode set “c” might comprise, e.g., electrode contacts 42 a, 42 band 42 c, all as cathodes, combined with electrode contact 42 h, as ananode. Likewise, electrode set “d” might comprise electrode contacts 42b and 42 c, both as anodes, combined with electrode contacts 42 a and 42d, both as cathodes. It is to be emphasized that such electrode sets areonly exemplary of some of the types of electrode sets that could beselected.

As seen in FIG. 3A, the perception thresholds for electrode sets “a”,“b”, “c”, and “d” are measured to be 1 mA, 2 mA, 2 mA and 3 mA,respectively. Similarly, the maximum tolerable thresholds for electrodesets “a”, “b”, “c”, and “d” are measured to be 9 mA, 7 mA, 8 mA and 6mA, respectively. A key feature of the present invention is that thesethreshold data, once measured during an initial set up period, are“equalized” so that the equalized threshold levels may thereafter beused as further testing continues to determine other optimum stimulusparameters and electrode configurations, and as subsequent use of theoptimum stimulus parameters goes forward.

Equalization is achieved, in accordance with the invention, as shown inFIG. 3B. As seen in FIG. 3B, the perception and maximum tolerablethreshold data gathered in FIG. 3A is mapped into unit-less “magnitudelevel” settings. The magnitude level settings are preferably representedby a number, e.g., the numbers 0 through 10. Alternate representationsof the magnitude level may also be employed, e.g., a visualrepresentation, such as bars on a bar chart, or color or gray-scaledensity of graphical symbols, or the like. In FIG. 3B, magnitude level 1is defined as the perception threshold of each electrode set. Thus,magnitude level 1 is 1 mA for electrode set “a”, 2 mA for electrode set“b”, 2 mA for electrode set “c”, and 3 mA for electrode set “d”.Magnitude level 10 is defined as the maximum tolerable threshold of eachelectrode set. Thus, magnitude level 10 is 9 mA for electrode set “a”, 7mA for electrode set “b”, 8 mA for electrode set “c”, and 6 mA forelectrode set “d”. The magnitude levels between the magnitude levels 1and 10 are then computed to be an appropriate value that lies betweenthe perception threshold and the maximum tolerable threshold, with themagnitude of the stimulus increasing as the number of the magnitudelevel increases. For example, the currents associated with magnitudelevels 2-9 for electrode set “a” may be determined by assuming a linearincrease. That is, assuming a linear relationship, there are eightmagnitude level slots that span from 1 mA to 9 mA in equal increments of8/9 mA, or approximately a 0.89 mA increase for each step up in themagnitude level. Thus, with this assumption, magnitude level 2 forelectrode set “a” would be set to 1.89 mA, magnitude level 3 to 1.89mA+0.89 mA=2.78 ma, and so on, up to magnitude level 10 which is 9 mA.

It should be pointed out that other relationships, i.e., other thanlinear, may also be assumed for the magnitude level values for eachelectrode set. Some possible relationships are explained in more detailbelow, and have particular applicability where more than one electrodeserves as an anode and/or cathode.

It should also be pointed out that, while the term “magnitude level” isused herein to describe the unit-less number that is assigned touniformly denote the stimulus level between the perception and maximumtolerable thresholds for each electrode set, the term “amplitude level”could also be employed inasmuch as it is usually the amplitude which isadjusted in order to change the energy associated with a given stimuluspulse. As is known in the art, however, pulse width also can be used toincrease or decrease the energy content of a stimulus pulse. Hence,because either pulse amplitude or pulse width can affect the energycontent of the stimulus pulse, the term “magnitude” is used herein as ageneric term intended to encompass pulse amplitude and/or pulse widthand/or other stimulus parameters that affect the energy content of thedelivered electrical stimuli.

Thus, as shown in FIG. 3B, the equalization process involves mapping theraw perception and maximum tolerable threshold data from FIG. 3A (orother threshold data) to the form shown in FIG. 3B, with the perceptionthreshold for each electrode set comprising a first magnitude level,e.g., magnitude level 1, and with the maximum tolerable threshold foreach electrode set comprising a second magnitude level, e.g., magnitudelevel 10. The available magnitude levels between the first and secondmagnitude levels are then assigned stimulation values that range fromthe perception threshold value to the maximum tolerable threshold valuein accordance with a prescribed relationship, e.g., a linearrelationship.

It should also be pointed out that, once some basic measurement data hasbeen taken for a suitable sample size of electrode sets, it may not benecessary to measure the minimum perception threshold level and themaximum tolerable threshold level for each electrode set. Rather, asingle threshold point may be measured, e.g., a most comfortablesensation level, which (through empirical or other testing may bedetermined to be a magnitude level 4 or 5), and thereafter the maximumtolerable threshold level (magnitude level 10) and minimum perceptionthreshold level (magnitude level 1), and all of the intermediatemagnitude levels may be estimated.

Additionally, it should be noted that not all electrode configurationsneed to be measured to determine threshold levels. Rather, a subset ofthe electrode configurations may be measured, and the magnitude leveldata for remaining (unmeasured) electrode configurations may thereafterbe estimated. For example, assume an in-line electrode array 42 of thetype shown in FIG. 1 having eight electrode contacts 42 a, 42 b, 42 c, .. . 42 h. Further, assume a monopolar electrode configuration. The onlythreshold measurements that may be needed are for the end electrodes,i.e., electrode 42 a and the reference electrode 32, and the electrode42 h and the reference electrode 32. Once the threshold data for theseend electrodes have been measured, then corresponding threshold data maybe estimated for the remaining monopolar electrode configurations. Forexample, suppose the minimum perception threshold for electrode 42 hwith respect to the reference electrode 32 is 2 mA, while the minimumperception threshold for electrode 42 a with respect to the referenceelectrode 32 is 3.75 ma. The minimum perception thresholds for theremaining monopolar electrode configurations may then be estimatedusing, e.g., a linear extrapolation approach, wherein the minimumperception threshold for electrode 42 b would be 3.5 mA, electrode 42 cwould be 3.25 mA, electrode 42 d would 3 mA, electrode 42 e would be2.75 mA, electrode 42 f would be 2.5 mA, and electrode 42 g would be2.25 mA. Estimation methods other than linear extrapolation may also beused. Additionally, once monopolar threshold measurements have beendetermined, threshold measurements for other configurations, such asbipolar, tripolar, or multipolar, may readily be estimated usingappropriate models of the stimulation circuit, as are known in the art.

Turning next to FIG. 4, there is shown a high level flow chart thatdepicts a method of programming a neural stimulator in accordance withthe teachings of the present invention. The goal of such method is toascertain optimum stimulus parameter values that may be programmed intothe neural stimulator. As seen in FIG. 4, a first step (block 50) of themethod involves selecting the number of electrode sets, N_(MAX), thatare to be used. Some of the electrode sets may include a singleelectrode paired with the reference electrode (a monopolarconfiguration), or two electrodes paired together (a bipolarconfiguration). While a large number of electrode sets couldtheoretically be used, in practice there will generally be a moremanageable number of electrode sets to use based upon the particularapplication involved and the symptoms experienced by the patient. Alsoincluded as an initial step is to set a pointer N to an initial value,e.g., to set N=1. Then, for a first electrode set (N=1), the perception(or other) threshold is determined and stored (block 52). Additionally,if possible, the maximum tolerable threshold is also determined andstored (block 54); although, as indicated above, a single thresholdmeasurement may be all that is required in some instances. This processis repeated for each electrode set (blocks 56, 58) until all of thethreshold data has been obtained for each defined electrode set. Asindicated above, in some instances, once some initial threshold data hasbeen obtained, the remaining data may be estimated. The gathering ofsuch threshold data thus creates a data set similar to that shown inFIG. 3A.

Next, after the threshold data has been gathered (measured and/orestimated) for each electrode or electrode set, the data is mapped(equalized) to magnitude levels (block 60), with a first levelcorresponding to the perception threshold, and with a second levelcorresponding to the maximum tolerable threshold. The mapping of suchthreshold data thus creates a data set similar to that shown in FIG. 3B.As part of this mapping or equalizing function, the available magnitudelevel numbers between the perception level and the maximum tolerablethreshold are assigned to current stimulus magnitudes that are computed,or otherwise determined, so that the energy content of the appliedstimulus increases as the “magnitude level” increases from theperception level to the maximum tolerable level.

Once the magnitude levels for each electrode or electrode set have beendetermined, the IPG 30 is controlled so as to sequence through selectedelectrode sets, or combinations of electrodes, at various magnitudelevels in order to determine which combinations of electrodes andstimulus parameters produce a desired result (block 62). For a spinalcord stimulator, the desired result will typically be an area or zone ofparesthesia that masks or blocks an area or zone of pain.Advantageously, such sequencing may, for the most part, be carried outunder automatic control of the IPG 30 and the external programmer 20.Any of numerous different types of sequencing algorithms may be used forthis purpose. By adjusting the magnitude of the applied stimulus interms of the “magnitude level” number, rather than the actual currentvalue, the testing or fitting process (i.e., the process of trying tolocate the optimum stimulation parameters and electrode set) may becarried out much quicker and safer than has heretofore been possible.Further, the system may continuously or systematically or under manualcontrol “jump” between two or more electrode sets, thereby allowing thepatient to compare which of the two or more electrode sets produces thebest (most efficacious) result.

EXAMPLE

In this example, a single in-line electrode array having eight electrodecontacts, as shown in FIG. 1, was used. Threshold data measurements,including perception and maximum tolerable, are made for each electrodein both a monopolar and a bipolar configuration. Magnitudes for eachelectrode are then equalized by assigning magnitude levels of “1” forthe perception threshold, and “10” for the maximum tolerable threshold.Each electrode has two stimulation magnitude ranges that are linearlyapplied depending on the number of cathodes and anode separation. Thefollowing relationships and definitions are used in this process:

X=Magnitude Level (0-10), where 0 level=0 mA.

I=mA for electrode (n).

P=Perception Threshold, mA.

M=Maximum Threshold, mA.

E=number of cathodic electrodes in a group.

If X=1, then I=P;

If X=10, then I=M;

If X=0, then I=0;

and for all other values of X, $\begin{matrix}{I = {\frac{\left( {M - P} \right)*\left( {X - 1} \right)}{9E} + \frac{P}{E}}} & (1)\end{matrix}$

The field location (stimulus location) is movable. That is, one or moreselected cathodes and anodes may be moved as a group by pressing up/downon a suitable programmer control unit, e.g., the programmer 20 (FIG. 1).The orientation of anodes and cathodes with respect to one anotherremains constant while the group or set of electrodes is moved up ordown among the possible programmable electrodes. Magnitude Level Xremains constant for all electrodes that become selected by the up/downinput, while individual current levels are input for each selectedelectrode based on the formula presented above in Equation (1).

The size of a “virtual” cathode may be adjusted by increasing ordecreasing the number of adjacent electrodes assigned to the “virtual”cathode. Increasing the size of a “virtual” cathode (or “virtual” anode)by increasing the number of adjacent electrodes with an up/down inputwhile maintaining a constant magnitude (amplitude) level causes currentsummation which may create an increased perception level. That is, whilethe magnitude (amplitude) level is maintained at a constant for the“virtual” cathode, the current values associated with this constantmagnitude level would be distributed among the individual electrodes asthey are added or subtracted into the “virtual” cathode. This means thatthe respective current values (mA) are applied to the distributed levelas follows: $\begin{matrix}{X = {E + \frac{\left( {I_{1} - P_{1}} \right)*9}{E\left( {M_{1} - P_{1}} \right)} + {\frac{\left( {I_{2} - P_{2}} \right)*9}{E\left( {M_{2} - P_{2}} \right)}\ldots \quad \frac{\left( {I_{n} - P_{n}} \right)*9}{E\left( {M_{n} - P_{n}} \right)}}}} & (2)\end{matrix}$

As required, one or more constants may be added to Eq. (2) when theeffect of summing the amplitudes at the electrodes is not perceived as aconstant level.

The current field may also be focused. Focus occurs by selecting anodesto create near and far field current paths. When the anodes are adjacentto the cathodes, the bipolar thresholds are used. In monopolar, themonopolar thresholds are used. As the anodes move away from thecathodes, i.e., as the separation distance between the anodes andcathodes increases, a linear range is extrapolated between the bipolarand monopolar threshold ranges.

FIG. 5 is a graph that illustrates the current distribution between twoelectrodes, designated electrode 1 and electrode 2, as well as thecurrent sum realized when the current flow through electrode 1 is summedwith the current flow through electrode 2, for magnitude (amplitude)levels ranging between 1-10. FIG. 6 depicts the numerical dataassociated with the graph of FIG. 5.

FIG. 7 illustrates patient threshold data in tabular and graphical formfor various electrodes associated with the example, and furtherillustrates the concept of current summing when more than two electrodesare included within the stimulation group.

By way of example, FIG. 8 shows the current and voltage potentialdistributions obtained at various points along an in-line electrodeassociated with a stimulus pulse that is applied between electrode two(cathode) and electrode seven (anode) of an in-line electrode havingeight electrodes numbered 0 through 7. Electrode 0 is the most distalelectrode, and thus electrode two (cathode) corresponds to electrodecontact 42 c in FIG. 1, and electrode seven (anode) corresponds toelectrode contact 42 h in FIG. 1.

Still by way of example, FIG. 9 similarly shows the current and voltagepotential distributions obtained at various points along an in-lineelectrode associated with a stimulus pulse applied between electrodesone, two and three (cathodes) and electrode seven (anode) of an in-lineelectrode having eight electrodes numbered 0 through 7.

As described above, it is thus seen that the present invention providesa system and method for programming the magnitude of electrical stimuligenerated and applied by a neural stimulator. e.g., a spinal cordstimulator. The electrical stimuli are applied through selectedgroupings of individual electrode contacts of a multi-electrode-contactelectrode array attached to pulse generation circuitry as eithercathodes or anodes. The electrode array is implanted so that theindividual electrode contacts are in contact with the body tissue to bestimulated. Stimulating electrical current pulses, defined by aprescribed set of stimulus parameters, e.g., pulse amplitude, pulsewidth and pulse repetition rate, are generated and applied to theselected electrode contacts so as to flow from the anode electrodes tothe cathode electrodes. As the current pulses flow through the bodytissue, the electrical current causes the neural stimulator to carry outits intended function, e.g, triggering a desired neural response orblocking an undesired neural response. A programming system or method isadvantageously included as a key part of the invention whereby theperceived magnitude of the applied stimuli is equalized in order toenable quick, automated, and/or interactive selection of the stimulationparameter values that are used by the stimulator.

As further described above, it is seen that the present inventionprovides the ability to dynamically switch between electrode sets whilethe stimulation is continuously applied. Such “jumping” or dynamicallyswitching allows immediate comparisons of stimulation to be made withouthaving to reset the stimulation magnitude, and while maintaining aconstant perception of intensity or magnitude of stimulation ascontrolled by unit-less magnitude levels, thereby avoiding over or understimulation.

As also described above, it is seen that the invention does not requirethat threshold measurements be taken for all possible electrodeconfigurations. Rather, a subset of the possible electrodeconfigurations may be measured, and from such measurements estimates maybe made of the unmeasured thresholds with relative accuracy.

As additionally described above, it is seen that the invention includesthe ability to store adjustments made to stimulation levels forestimated electrode thresholds so that the system learns corrections tothe estimated equalized levels.

Further, it is seen that the invention allows adjustment and/orcompensation of the perception stimulus when the pulse width is changed,or when any other combination of stimulus parameters is changed thataffect the energy of the delivered stimulus pulse.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. A method for operating an implantable neuralstimulator, wherein the implantable neural stimulator includes aprogrammable pulse generator circuit housed within a sealed case, anelectrode array connected to the neural stimulator, the electrode arrayhaving a plurality of electrode contacts, and wherein variouscombinations of the electrode contacts comprise sets of electrodesthrough which an electrical stimulus is applied, wherein the methodcomprises: (a) measuring and recording at least one perception point forat least a selected subset of the sets of electrodes; (b) estimating andrecording perception points for unmeasured sets of electrodes; and (c)mapping recorded and estimated values to magnitude levels for use withdesignated stimulation parameters in a set of stimulus parameters thatare programmed into said implantable neural stimulator.
 2. The method ofclaim 1 further including storing, calculating, and automaticallyadjusting magnitude levels when a given set of electrodes is selected asthe electrode set through which an electrical stimulus is to be applied.3. The method of claim 1 wherein mapping recorded and estimated valuesto magnitude levels comprises mapping the recorded and estimated valuesto unit-less magnitude numbers.
 4. The method of claim 3 furtherincluding defining a set of unit-less magnitude numbers corresponding toeach set of electrodes, a first magnitude number being mapped to a firststimulus current producing a minimum perception threshold level, and asecond magnitude number being mapped to a second stimulus currentproducing a maximum tolerable threshold level.
 5. The method of claim 4wherein defining the set of unit-less magnitude numbers comprisesdefining a set of at least three unit-less magnitude numbers for eachset of electrodes, the first magnitude number corresponding to the firststimulus current that produces the minimum perception threshold level,the second magnitude number corresponding to the second stimulus currentthat produces the maximum tolerable threshold level, and a thirdmagnitude number corresponding to a third stimulus current greater thanthe first stimulus current and less than the second stimulus currentthat produces a comfortable threshold level.
 6. The method of claim 4wherein defining the set of unit-less magnitude numbers comprisesdefining a set of ten unit-less magnitude numbers for each set ofelectrodes, each unit-less magnitude number corresponding to arespective stimulus current, the respective stimulus currents rangingfrom the first stimulus current associated with the minimum perceptionthreshold level and the second stimulus current associated with themaximum tolerable threshold level.
 7. The method of claim 3 furtherincluding applying a stimulus through a first set of electrodes having amagnitude defined by a selected magnitude number, applying a stimulusthrough a second set of electrodes having a magnitude defined by thesame selected magnitude number, comparing the result of the stimulusapplied through the first set of electrodes with the result of thestimulus applied through the second set of electrodes in a comparisontest, and selecting stimulus parameters for programming into theimplantable neural stimulator as a function of the comparison test. 8.The method of claim 1 wherein estimating and recording perception pointsfor unmeasured sets of electrodes comprises: determining electrode setsfor which perception points have not been measured; estimatingperception points for the unmeasured sets of electrodes by determiningthe perception points for the unmeasured sets of electrodes as afunction of the perception points of the measured sets of electrodes;and recording the estimated perception points.
 9. The method of claim 8further including calculating the perception points comprisesextrapolating the perception points for the unmeasured sets ofelectrodes from the perception points from the measured sets ofelectrodes.
 10. An implantable neural stimulator comprising: (a) pulsegeneration circuitry housed within a sealed case, said pulse generationcircuitry being adapted to generate electrical stimuli in accordancewith programmed stimulus parameters; (b) an implantable electrode arrayhaving a multiplicity of electrode contacts, wherein each of themultiplicity of electrode contacts is selectively connected to the pulsegeneration circuitry, and wherein the electrode contacts are grouped inselected electrode sets through which the electrical stimuli generatedby the pulse generation circuitry is directed, each electrode setincluding a plurality of the electrode contacts, wherein one of theelectrode contacts comprises the sealed case wherein the pulsegeneration circuitry is housed; (c) a known threshold level for at leasta plurality of the multiplicity of electrode contacts, where said knownthreshold levels are expressed in units of current or voltage; and (d)an equalizer circuit that equalizes the threshold levels to unit-lessmagnitude levels.
 11. The neural stimulation system of claim 10 whereinthe equalizer circuit further estimates threshold levels for electrodecontacts not initially known.
 12. The neural stimulation system of claim11 further wherein the neural stimulator automatically adjusts themagnitude of the stimulus applied through a selected electrode set to avalue determined from the unit-less magnitude levels for the selectedelectrode set.
 13. A system for determining an optimal set of stimulusparameters to be programmed into implantable pulse generation circuitry,the implantable pulse generation circuitry being housed within a sealedcase, the pulse generation circuitry being adapted to generateelectrical stimuli in accordance with programmed stimulus parameters;and further wherein an electrode array having a plurality of electrodecontacts is selectively connected to the pulse generation circuitry;said system comprising: at least one measured perception threshold for aselected set of the plurality of electrode contacts, where said at leastone perception threshold is measured in units of current or voltage; anequalizer that equalizes the at least one measured perception thresholdfor the selected set of electrode contacts to a unit-less magnitudelevel, and that estimates perception thresholds and correspondingmagnitude levels for sets of electrode contacts where perceptionthresholds have not been measured; and means for automatically adjustingthe magnitude of the electrical stimuli applied by an implantable neuralstimulator through a selected set of electrode contacts as a function ofunit-less magnitude levels associated with the selected set ofelectrodes.
 14. The system of claim 13 wherein the equalizer comprises:memory circuitry wherein the perception threshold is stored; mappingmeans for mapping the perception threshold of a given set of electrodecontacts to a first number magnitude level, wherein when a stimulushaving the first number magnitude level is selected to be applied to aselected set of electrode contacts, the set of electrode contactsreceives a stimulus having a magnitude equal to the perceptionthreshold.
 15. The system of claim 14 wherein there are at least twoperception thresholds for each selected set of the plurality ofelectrode contacts, one perception threshold comprising a minimumperception threshold level, and another perception threshold comprisinga maximum tolerable threshold level; and wherein the mapping means mapsthe minimum perception threshold level to the first number magnitudelevel, and maps the maximum tolerable threshold level to a second numbermagnitude level; wherein the selected set of electrode contacts receivesa stimulus having a magnitude equal to or near the minimum perceptionthreshold level when the first number magnitude level is selected, andwherein the selected set of electrode contacts receives a stimulushaving a magnitude equal to or near the maximum tolerable perceptionthreshold level when the second number magnitude level is selected. 16.The system of claim 15 wherein the first number magnitude levelcomprises a low number, and the second number magnitude level comprisesa high number, and further wherein a plurality of number magnitudelevels exist greater than the low number and less than the high number,and wherein the pulse generation circuitry generates a stimulus having amagnitude greater than the minimum perception threshold level and lessthan the maximum tolerable threshold level when one of the numbermagnitude levels between the low and high numbers is selected as themagnitude level of the stimulus to be applied through a given set ofelectrode contacts.
 17. The system of claim 16 wherein the low numbercomprises the number “1”, and the high number comprises the number “10”,and further wherein any of the number magnitude levels “1”through “10”is selected as the magnitude level of the stimulus that is to be appliedthrough the selected set of electrode contacts.
 18. The system of claim17 wherein the number “0” is selected as a magnitude level, and whereinthe magnitude of a stimulus having a magnitude level “0” is zero or off.19. The system of claim 13 wherein at least one of the plurality ofelectrode contacts comprises a reference electrode contact locatedintegrally with the sealed case of the implantable neural stimulator.20. A neural stimulation system comprising: implantable pulse generationcircuitry housed within a sealed case, said pulse generation circuitrybeing adapted to generate a stimulus pulse in accordance with programmedstimulus parameters; an electrode array having a multiplicity ofelectrode contacts, wherein each of the multiplicity of electrodecontacts is selectively connected to the pulse generation circuitry;wherein a minimum perception threshold and a maximum tolerable thresholdfor each of the multiplicity of electrode contacts is known ordeterminable, wherein said thresholds are measured in units of currentor voltage; and an equalizer that equalizes the minimum perceptionthreshold and the maximum tolerable threshold to unit-less magnitudelevels; and wherein the pulse generation circuitry, upon command,automatically adjusts the magnitude of the stimulus pulse appliedthrough a selected combination of the multiplicity of electrode contactsto a level referenced to the unit-less magnitude levels.
 21. The neuralstimulation system of claim 20 wherein the equalizer comprises: memorycircuitry wherein the minimum perception threshold and the maximumtolerable threshold are stored; a mapping circuit that maps the minimumperception threshold of each electrode contact to a first numbermagnitude level and that maps the maximum tolerable threshold to asecond number magnitude level, wherein when a stimulus having the firstnumber magnitude level is selected to be applied to a selectedcombination of electrode contacts, the selected combination of electrodecontacts receives a stimulus having a magnitude equal to or near theminimum perception threshold, and when a stimulus having the secondnumber magnitude level is selected to be applied to a selectedcombination of electrode contacts, the selected combination of electrodecontacts receives a stimulus having a magnitude equal to or near themaximum tolerable threshold.
 22. The neural stimulation system of claim21 wherein the first number magnitude level comprises a low number, andthe second number magnitude level comprises a high number, and furtherwherein a plurality of levels exist greater than the low number and lessthan the high number, and wherein the implantable neural stimulatorgenerates a stimulus having a magnitude greater than the minimumperception threshold and less than the maximum tolerable threshold whenone of the levels between the low and high numbers is selected as themagnitude level of the stimulus to be applied to a selected combinationof electrode contacts.
 23. The neural stimulation system of claim 22wherein the low number comprises the level “1”, and the high numbercomprises the level “10”, and further wherein any of the levels “1”through “10” is selected as the magnitude level of the stimulus appliedthrough a selected combination of electrode contacts; and wherein theimplantable neural stimulator generates a stimulus having a magnitudegreater than the minimum perception threshold measurement and less thanthe maximum tolerable threshold measurement when one of the levels “2”through “9” is selected as the magnitude level of the stimulus to beapplied to the selected combination of electrode contacts.